Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which comprises an illumination device and a projection lens. The image of a mask (=reticle) illuminated by way of the illumination device is in this case projected by way of the projection lens onto a substrate (e.g., a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In the lithography process, undesired defects on the mask have a particularly disadvantageous effect since they can be reproduced with every exposure step. A direct analysis of the imaging effect of possible defect positions is thus desirable in order to minimize the mask defects and in order to realize a successful mask repair. Therefore, there is a need to measure or to qualify the mask rapidly and simply, to be precise as far as possible under the same conditions such as are really present in the projection exposure apparatus.
For this purpose, it is known, in a mask inspection apparatus, to record and evaluate an aerial image of a segment of the mask. For recording the aerial image, in this case the structures to be measured on the mask are illuminated by an illumination optical unit, wherein the light coming from the mask is projected via an imaging optical unit onto a detector unit and detected.
In order to carry out the measurement of the mask if possible under conditions analogous to those in the projection exposure apparatus, in the mask inspection apparatus the mask is typically illuminated in a manner identical to that in the projection exposure apparatus, wherein in particular the same wavelength, the same numerical aperture and also the identical (if appropriately polarized) illumination setting are set in the mask inspection apparatus.
However, in practice a problem results from the fact that in the imaging optical unit of the mask inspection apparatus, the imaging of the mask onto the detector unit—unlike the imaging on the wafer that is carried out in the projection exposure apparatus—does not take place in reduced fashion, but rather in greatly magnified fashion. The thus greatly different numerical aperture present in the respective projection or imaging optical unit (said numerical aperture being almost zero in the imaging optical unit of the mask inspection apparatus) has the consequence that the imaging on the wafer that takes place in the lithography process differs significantly from the imaging onto the detector unit that takes place in the mask inspection apparatus with regard to vector effects that occur. In this case, “vector effect” should be understood to mean the polarization dependence of the interference of the electromagnetic radiation that takes place in the respective image plane.
In order to take account of the above problem and to determine the vector effects that occur in the microlithographic projection exposure apparatus and to take them into account in the aerial image generation, it is known, in particular, to carry out a plurality of individual imagings with the mask inspection apparatus, during which individual imagings different polarization-optical components are positioned in the illumination and/or imaging optical unit and the correspondingly generated images are combined with one another and subjected to computation. However, the procedure described above requires a comparatively high time expenditure.
With respect to the prior art, reference is made merely by way of example to DE 10 2004 033 603 A1, DE 10 2004 033 602 A1, DE 10 2005 062 237 A1, DE 10 2007 009 661 A1 and EP 1 615 062 B1.